GUIDANCE ON THE USE OF CRASH COST ESTIMATES

The preceding two sections provided a discussion of possible issues with the cost estimates developed and guidance related to how some of these issues might be overcome. This section provides some limited general guidance on the use of crash cost estimates in safety studies.

Comprehensive versus Human Capital (Economic) Cost Estimates

As noted earlier, both comprehensive and human capital cost estimates are provided in the accompanying tables. Comprehensive cost estimates include not only the monetary losses associated with medical care, other resources used, and lost work, but also nonmonetary costs related to the reduction in the quality of life. Since human capital costs do not capture the full burden of injury, comprehensive costs are generally used in analyses related to not only safety issues, but also other public health issues (e.g., effects of the environment on health) and by other nontransportation federal and state agencies. Thus, it is recommended that the comprehensive cost estimates provided in the tables be used.

Choice of Cost "Level"

By developing six different levels of crash-cost estimates, this study has provided future users with a significant amount of flexibility in what crash-cost estimates to use in a given study. As described above, a component of the decision concerning which level of estimate to use will be the stability of the cost estimates for the crash types being studies-whether or not there are a large number of "flagged" estimates.

However, the most important determinate of the cost level to be used is the size of the crash data samples under study, more specifically, the number of fatal and serious-injury crashes available for study. If the analysis involves a national sample of data (e.g., an analysis using multiple years of FARS or GES data), then it may be possible to use the more detailed crash-cost estimates where each fatal crash is assigned a cost (i.e., Level 1estimates). However, in most safety studies, the number of fatal crashes in a given analysis "cell" (e.g., a specific type of crash at signalized intersections) is limited, and often the presence of one of two additional fatal crashes can greatly inflate the cost for the entire cell and disproportionately affect the economic results of the study. Hall (1998)(42) noted in crash-cost research conducted for the State of New Mexico that not only are such fatalities somewhat "random" in any crash sample, but that the main factors determining whether an injury is a fatality rather than a severe injury are not very likely to be affected by roadway-related treatments. They are more likely to be related to occupant age, restraint use, type and size of vehicles involved, etc. He therefore argues that such small numbers of fatalities should not be allowed to affect decisions on roadway-based treatments such as those often of interest to FHWA.

In summary, the decision concerning which level of cost estimate to use in a given study will have to be made by the researcher after review based on the nature of the data-specifically the number of fatal crashes in the data set-and the stability of the comprehensive cost estimates provided for the types of crashes under study. Generally, researchers should use the highest (least detailed) cost level possible that can still provide information on the study question of interest. For example, the research information needed may require that specific crash types be analyzed, but may not require categorization by speed limit. Other studies may not require categorization by crash type, and available sample sizes of fatal crashes may allow the use of crash-cost for each KABCO severity level (i.e., as in Level 5 estimates).

Modifying Crash Cost Estimates for Specific Years

The cost estimates developed in this study use 2001 dollars. If human capital crash costs are required for another year, the recommended adjustment procedure is to multiply the human capital costs provided in the tables by a ratio of the Consumer Price Index (CPI)-all items (CPI) for the year of interest divided by the CPI for 2001.(42) If comprehensive crash costs are required for another year, a two-step process is recommended. First, the human capital portion of each unit cost is adjusted as described above. Then, the difference between the comprehensive cost and the human capital cost for a given unit crash cost should be multiplied by a ratio similar to that for human capital costs. However, instead of using the CPI, one should use the Employment Cost Index, not seasonally adjusted, total compensation, total private industry.(43) Adding the two components yields updated comprehensive costs. This procedure should provide adequate cost estimates for roughly 5 years or until the next major DOT update of unit crash cost data and methods.